Respiratory systems provide breathable gas, such as oxygen, anesthetic gas, and/or air directly to a patient's mouth, nose, or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit. An expiratory limb hose or conduit may be provided to allow air to expel from the patient.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a humidification system having a chamber for holding water and a heater unit to which the chamber may be releasably mounted. The heater unit includes a heater, which may be comprised of one or more heating elements and a metal plate defining a hot plate. A wall of the chamber, such as the bottom surface of the chamber, is thermally conductive and in thermal contact with the hot plate of the heater, to thus heat the water in the chamber. The chamber may be manually refillable, or there may be a water source to selectively fill the chamber as it empties. The breathable gas is coupled to the chamber and is passed through the chamber to be heated and humidified. Examples of heater unit and chamber arrangements are shown in U.S. Pat. Nos. 6,988,497 and 5,943,473. The inspiratory limb carries the heated and humidified gas to the patient and the expiratory limb, if present, carries exhaled air and possibly other gases from the patient. Either or both of the inspiratory and expiratory limbs may be heated such as by heater circuits, which may be comprised of wires running through and along the hose or conduit interior. An example of a breathing circuit with heated limbs is shown in U.S. Pat. No. 6,078,730. In some settings, the limb(s) may not be heated.
Maintaining the desired temperature of gas(es) passing through this type of respiratory system may require adjusting the temperature of the heater in the heater unit and/or the heater circuits in the inspiratory and expiratory limbs in response to thermal feedback from the system. Thus, some respiratory systems include temperature probes at one or more locations, such as for sensing the temperature of the heated and humidified gas supplied to the patient. The temperature probes may be operatively coupled to the heater unit, which then adjusts the power levels to the heater and/or heater circuit(s) based at least in part on the measured temperatures. Current temperature probes for respiratory systems typically include a thermistor, which is packaged in a cylindrical container, such as a polyimide tube, and secured therein with epoxy, all held inside the plastic housing of the temperature probe by a potting compound. Lead wires are electrically coupled to and extend away from the thermistor to be electrically coupled to an associated temperature cable at an opposite end of the housing for electrically communicating with the heater unit.
In respiratory systems, the humidity level of the heated gas is quite high, typically at or near 100%. That high humidity level, coupled with the temperature of the heated gas, is believed to cause significant migration of moisture into the probe housing and against the thermistor. Moisture thus builds up and causes the thermistor to experience electronic drift and/or mechanical compression, such that the temperature readings therefrom become inaccurate and unreliable. In prior thermistor-based probes for respiratory systems, the moisture build-up might happen so rapidly as to render the device generally useless after only a matter of days or weeks. Much longer useful life is necessary. One consideration to reduce the migration of moisture to the thermistor is to encapsulate the thermistor container (or the thermistor if no container is used) in a large bead of glass or epoxy. While the large bead might be expected to reduce moisture problems for the thermistor and thus extend the useful life of the probe, it is believed that such an approach dramatically reduces the temperature response of the thermistor. The temperature readings may thus unduly lag actual changes in temperature, such that the heater unit may not provide adequate temperature regulation of the heater in the heater unit and/or the heater circuit(s) in the breathing circuit limb(s).